But in October, he announced he would be quitting the sci-fi show later this year.

Lots of speculation

Since then, loads of people have been wondering who he will regenerate into.

Some Who-fans even think the new Doctor could be a woman, for the first time in the show's history.

Some actors being talked about to take over:

Paterson Joseph (pictured)

David Morrisey

James Nesbitt

John Simm

Head Of Drama BBC Wales Piers Wenger said: "We are so pleased to have been able to cast this person as the new Doctor.

"We believe the actor is going to bring something very special to the role and will make it absolutely their own - I just can't wait to tell everyone who it is - it has been a nail biting Christmas trying to keep this under wraps!".

Whatever happens, David Tennant's time as the Doctor isn't over just yet.

He's currently filming four special episodes of the show, which will go out this year and in early 2010.